Method of forming a dielectric oxide film on a metal strip



Jan. 13, 1959- J. B. BRENNAN 2,368,702

METHOD OF FORMING A DIELECTRIC OXIDE FILM ON A METAL STRIP Filed Nov. 4, 1952 2 Sheets-Sheet 1 IN V EN TOR.

JOSEPH B. BRENNAN;

ATTORNEYS Jan. 13, 1959 J, B, BRENNAN 2,868,702

METHOD OF FORMING A DIELECTRIC OXIDE FILM ON A METAL STRIP Filed Nov. 4. 1952 2 Sheets-Sheet 2 INVENTOR.

JOSEPH B. BRENNAN (9141M #QQAAW ATTORNEYS United States Patent METHOD OF FORMING A DIELECTRIC OXIDE FILM ON A METAL STRIP Joseph "B. Brennan, Cleveland, Ohio; Helen E. Brennan,

executrix of the estate of said Joseph B. Brennan, deceased Application November 4, 1952, Serial No. 318,618

3 Claims. (Cl. 204-28) This invention relates to improved method for ac- 'celerating the forming of metal electrode strip material for electrolytic cells and the formation of oxide films on such electrode strip material by, for example, heating the said electrode strip material and exposing it to oxidizing materials such as steam while so heated, combined with shortened time and less energy required for electro-formation action on the electrode strip.

The metallic strip material suitable for oxide treatment according to this invention may be in strip, wire, or foil form and it may be porous or non-porous.

Such strip may be made of or include aluminum, or titanium or other metals which will oxidize when exposed to steam, boric acid solution or oxygen yielding materials when the exposed metals are heated up to red heat but to less than their melting point.

The oxide films formed by this invention are dielectric to a degree generally dependent upon the metal used and the thickness of the oxide layer produced as well as other factors.

The general object of the invention is to provide a novel method of accelerating the filming of electrode strip material of the class described characterized by the initial thermo-chemical oxidation of the metal electrode strip with a subsequent electrolytic oxidation of the electrode strip.

Another object of the invention is to reduce the cost of producing metal electrode strips and especially to reduce the time and energy required to electro-form an oxide layer over the surface of the electrode strip of film-forming metal.

Another object of the invention is to accelerate the electrolytic formation of an electrode strip and to reduce the amount of electrical energy involved.

The foregoing and other objects and advantages of the invention will become more apparent as the specification proceeds.

For a better understanding of the invention, reference should be had to the accompanying drawings, wherein:

Fig. l is a section through apparatus for performing one portion of the method of the invention;

Fig. 2 is a vertical section of diagrammatic apparatus for performing a second part of the method of the invention; 1

Fig. 3 is a section, like Fig. 2, of a further modification of apparatus of the invention;

Fig. 4 is a side elevation of the apparatus of Fig. 4; and

Fig. 5 is an enlarged elevation on line 5-5 of Fig. 4.

Referring now to the details of the apparatus disclosed, 1 in Fig. 1 illustrates a coil of metal, for example porous aluminum strip, produced by conventional apparatus, which is fed as a strip over the outside of a ceramic porous roll 3 which roll revolves on a shaft 4 and is hollow. Details on the preparation of such porous metal strips may be found in my Patent No. 2,547,371. Strip 2 is fed under an asbestos, or other refractory porous belt or chain 5 which rides on pulleys or rolls 6.

2,868,702 Patented Jan. 13, 1959 Thus the strip 2 is firmly held to conform to the contour \of and against the surface of the drum 3 by the belt 5. Inside the hollow drum 3 is a high frequency coil 7 and also on top of the belt 5 is a similar high frequency coil 7a to create a 10,000-cycle field, for example, eifective to heat the metal strip 2 to red heat and maintain it at this temperature while in the high-frequency zone.

The roll 3 in conjunction with the belt 5 keeps the metal strip 2 from being heat-warped while in the highfrequency zone, due to the fact that the belt 5 maintains a holding pressure on the metal strip 2 in conjunction with the roll 3.

While the strip 2 is in the high frequency zone between the coils 7 and as it passes progressively therebetween, an oxidizing agent is fed from a suitable source through supply or feed openings 8 to the chambers 9 and therefrom through orifices 10 onto the surfaces of and into and through the interstices of the strip 2, so that an oxide film is produced on the strip .2 while in this zone and heated. The strip 2 then may be wound into a coil 12.

If desired, the strip 2 may be anodically polarized as it progressively leaves the enclosure and heating zone and led into an electro-formation electrolyte directly. The fluid supplied to the chambers 9 penetrates the porous ceramic roll 3 in the high frequency zone.

The high frequency coils 7 may be enclosed in a conventional dielectric 11 to prevent arcing due to moisture, and the orifices 10 are. formed in such dielectric. It is preferred to use dry, superheated steam or other suitable oxidizing agents for oxidizing the strip as it passes over the roll 3.

In any case, according to this invention, the hot strip metal is oxidized by oxygen gas, air, steam or other oxidizing materials while being heated and held in shape, and preferably moved along continuously. Boric acid or other electrolyte solution commonly employed in the electrolytic oxidation of aluminum may be used in place of the oxidizing agents mentioned.

Metal strips, oxidized in accordance with the present invention by chemical treatment thereof, may have the dielectric film formed thereon in any conventional manner, as by immersing the metal strip in an electrolyte bath comprising an aqueous solution of borax and boric acid, wherein the bath contains about .06% of borax and 8% of boric acid and the remainder pure water. Unidirectional electric current is caused to flow through such metal strips during the first oxidation step or subsequent thereto and enough amperes of electrical current are caused to flow through the strip and electrolyte bath when potential is applied thereto to form an oxide film thereon electro-chemically. The positive pole of the source of current is connected to the metal strip to be formed by conductive rolls in contact therewith by the practice of the invention. Preferably the temperature of the electrolyte is maintained at about to '90 degrees centigrade during the forming operation. When treating electrode strips in accordance With prior procedures, it has taken about 2 hours for the electro-formation of the desired dielectric film on a porous metal strip. By added use of thermal oxidation in accordance with the invention, the electro-formation time has been reduced to approximately 60 minutes, which figure is variable and average and dependent upon the metal being processed and theparticular properties desired in the finished electrode strip. Such a figure applies to film forming metal for use in an electrolytic condenser. Specifically, a porous electrode having a normal formation capacity of 50 microfarads 450 volts would take about two hours to bring it down to 1 milliampere leakage of current as a result of the electro-formation thereof, can have the electro-formation time required for processing such electrodes reduced to about 60 minutes, where such electrode has been chemically and thermally oxidized prior to the electro-formation thereof, all as disclosed by the present in n- In h fe eeeine c mp the s m nu ber of ampere volts would be applied per unit of time to the electrode being processed, in accordance with conventional practice and in accordance with the present invention. The cost of electro-forming as norm-ally practiced is about ten percent of the finished condenser and this invention saves at least 20% of the electroforming cost.

'The electro-formation operation or process of the invention may be practiced with other arrangements, and Fig. 2 shows one type of apparatus that may be used for this step of the present novel practice. Fig. 2 shows that the strip 2 of the invention may pass into a container 13 in which a suitable electrolyte is present for electro-forming the aluminum strip 2 to oxidize same and provide the desired dielectric film thereon. This strip 2 usually would pass over a guide roll 14 and then be threaded around a plurality of guide rolls 15 which are received within the tank 13 below the surface of the electrolyte contained therein. Electrical energy may be transmitted to the strip 2 in any desired manner and usually a contact roll 16 bears on the strip 2 to provide the desired direct current energy supply connection for the strip 2. A cathode 16a is suspended in the electrolyte to complete the electrical circuit and it is connected to the D. C. supply. In some instances, the contact roll 16 may bear upon the strip 2 prior to the chemical oxidation of such strip, so that a better electrical contact is made with the strip, where the strip 2 is continuously processed and directly moves to the tank 13 from the roll 3.

Another feature of a novel type of electro-formation acceleration process is disclosed in Fig. 3 wherein a tank 24 is provided and has a suitable electrolyte 25 positioned therein. A porous metal strip 26 which is to have a dielectric oxide film electro-formed thereon, passes over guide rolls 27 in being led to and from the tank 24, whereas a plurality of foraminous rolls 28 are suitably positioned within the tank 24 below the surface of the electrolyte 25. These foraminous rolls 28 may have center bores 29 provided therein with positioning sleeves or tubes 30 engaged with the ends of the bores 29 and being journalled in walls of the tank 24 by suitable bearings 30a. The sleeves 30 connect to conventional rotary couplings 31 from which conduits 32 extend. These tubes or conduits 32 connect to a center return tube 33 connected to the input side of asuitable pump 34 which is used to efiect a circulation of the electrolyte in the tank 24 and to aid in drawing the electrolyte through the pores in the strip 26 and the foraminous rolls 28 into the pores thereof. The liquid then is drawn into the tubes 32 and 33 from which it passes-to the pump 34and then is forced through a suitable tube 35 back into the tank 24 to complete its path of flow and maintain the electrolyte 25 at a desired constant level in the tank 24. By pumping electrolyte through thereof is accelerated, especially on the interior surface thereof, as more oxygen-bearing ions are brought into' contact with the surfaces of the strip to accelerate and produce the desired oxidation thereof, and with less electric energy consumption.

Suitable masks 28a are positioned over the surfaces of the rolls 28 to prevent the drawing of electrolyte through the portions of the rolls 28 not covered by the strip 26. These masks 28a are made from any imperviousmaterial inert in the electrolyte.

If necessary, suitab1e sealing or retainer means may be carried by the ceramic 11 to retain steam or other oxidizing medium fed to the openings 8 in contact with the adjacent surfaces of the belt 5 or roll 3 for passage to the strip 2.

The electrolytes referred to in the various steps and modifications of the invention all may be of the same composition, and the composition referred to hereinbefore is set forth by weight.

In another instance, an electrolyte containing 79.4 g. of boric acid, 1.1 g. or borax and 900 cc. of pure water had equal area strips of aluminum from a common base strip electro-formed therein. One strip was heated in air to a temperature just under its melting point and then electro-forrned at 240 volts D. C. for seven minutes. Such strip had a leakage of 4.4 milliamperes at 240 volts. The other strip, without prior oxidation, took nine minutes at 240 D. C. volts to be electro-formed to the same degree and leakage current at 240 D. C. volts as the first or pre-oxidized strip.

This application is a continuation in part of my copending application Serial No. 104,369, filed July 12, 1949 (now abandoned).

While in accord with the patent statutes I have specifically illustrated and described several manners of practicing my invention, it is to be particularly understood that I am not to be limited thereto or thereby, but that the scope of my invention is defined in the appended claims.

I claim:

1. A method of continuously producing an oxide film on the surface of thin flexible metallic strip comprising the steps of continuously moving the metal strip through a path, confining the metal strip to prevent distortion thereof as it is moved along the path, heating the metal when it is confined, exposing the strip to oxidizing conditions when it is heated and confined to a temperature in excess of 212 F. to form thereon an oxide film relatively thick as compared with the film formed by exposure to air at normal room temperatures, and thereafter electro-forming a dielectric oxide film on the metal strip to produce a unitary continuous oxide film having a high dielectric strength.

2. A method of forming a dielectric oxide film on the surface of a metal strip which comprises thermally oxidizing the surface of the metal at a temperature between 212 F. and below the melting point of the metal to form thereon an oxide film relatively thick as compared with the film formed by exposure to air at normal room temperature, and thereafter electro-forming a dielectric oxide film on the metal surface beneath said thermallyproduced oxide fil-m to produce a unitary continuous oxide film having a high dielectric strength.

3. In the formation of a dielectric oxide film on a porous metal strip, the steps of thermally oxidizing the surface of the metal strip at a temperature between 212 F. and the melting point of the metal to form thereon an oxide film relatively thick as compared with the film formed by exposure to air at normal room temperature, immersing the thermally oxidized porous metal strip in an oxidizing electrolyte, electro-forming a dielectric oxide film on said strip while immersed in said electrolyte and forcing said electrolyte through the porous metal strip during said electro-forming.

References Cited in the file of this patent UNITED ST. TES PATENTS 1,748,012 Dooley Feb. 18, 1930 1,853,437 Kuttner Apr. 12, 1932 2,304,073 Brennan Dec. 8, 1942 FOREIGN PATENTS 391,903 Great Britain May 11, 1933 

2. A METHOD OF FORMING A DIELECTRIC OXIDE FILM ON THE SURFACE OF A METAL STRIP WHICH COMPRISES THERMALLY OXIDIZING THE SURFACE OF THE METAL AT A TEMPERATURE BETWEEN 212* F. AND BELOW THE MELTING POINT OF THE METAL TO FORM THEREON AN OXIDE FILM RELATIVELY THICK AS COMPARED WITH THE FILM FORMED BY EXPOSURE TO AIR AT NORMAL ROOM TEMPERATURE, AND THEREAFTER ELECTRO-FORMING A DIELECTRIC 